Why is this mirror object not reflecting light onto another surface properly?
The roughness setting is insanely low, which makes the reflected light beam very narrow. If you increase the roughness, the light will be spread in a wider beam. This will, of course, also make the reflection of Suzanne somewhat blurry. In the image I've included, I have it set to .1, but I also tried .01 (10 times the .001 you have), and that made the interior of the cube visibly brighter.